OpsMgr Agent upgrade fails

I have just installed OpsMgr CU4 on my OpsMgr server, but when Approving agent upgrade in Pending Management, one server comes up with:

"The MOM Server could not start the MOMAgentInstaller service on computer "name" in the time."

This was the only server having having the problem. This server is running Windows 2008 with firewall enabled, but I have other servers with same configuration (also W2k8 w. FW enabled).
BUT... it turned out that IT WAS the firewall that was the problem.

Deleting the rule "MOM Agent Installer Service" solved the problem.